In a nuclear reactor, moderators slow down the neutrons which come out in a fission process. The moderator used have light nuclei. Heavy nuclei will not serve the purpose, because:

Fusion processes, like combining two deuterons to form a \(\mathrm{He}\)-nucleus are impossible at ordinary temperatures and pressure. The reasons for this can be traced to the fact:

1. (a), (c)
2. (a), (d)
3. (b), (d)
4. (a), (b)